Pokémon Go's network traffic has just leaked one of the Generation 8 Pokémon numbers and a placeholder sprite to accompany it.
The new Pokémon were referred to as "new Kecleon icon" but Unity files are tagged with Pokémon 891 and the design isn't related to Kecleon.
There is also the possibility that these are some kind of placeholders but the 3D models are well made and there is even a shiny form! At present there isn't many detail for this Pokémon so we will have to wait some official statement.
What do you think of these two?

This unknow Pokémon has briefly appeared in Pokémon Go for half an hour across the Asia-Pacific region (and Europe) following the conclusion of the Chikorita event. If caught the Pokémon turned into Ditto.
Kento Suga, the head of Pokemon GO‘s marketing in Japan made a Tweet saying "???" so maybe the Pokemon’s appearance was a marketing ploy.

I'd like to believe that this pokemon is just a marketing tool. Kinda like how zygarde cells and cores were teased at the end of a movie last time.
Zygarde cells can't be used in battle in the main series. Somehow, I wish this Pokemon will be the same.

It's kinda odd how we were teased a new Pokemon like this via GO. Don't we usually get exposed to the new pokemon in their offical artwork through scans or concept art? Probably the first time a pokemon was leaked through its 3D model.

It's not that strange that this Pokémon was teased via GO... you need GO and Let's Go to get this new Pokémon (and they still haven't explained if you will obtain it only in GO or if you will be able to transfer it to Let's Go)

Don't we usually get exposed to the new pokemon in their offical artwork through scans or concept art? Probably the first time a pokemon was leaked through its 3D model.

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No, not always we get exposed to new Pokémon with their official artwork... a lot of Pokémon are leaked a lot of time before.... for example in SM we got the 3 starters final form in a leak months before the release and in the demo they hacked everything and found all the 3D models (and their shiny form) and the Pikachu with hats (revealed ini March and available for distribution (in Europe) in September... a year later!) Oh! And they found even the Beast Ball!
Same for Vivillon they found the 2 exclusive patters before the official reveal... the same happened with Hoopa, Diancie, Volcanion and Mega Latios/Latias (the Mega Lati were available only in ORAS) and don't forget Floette Eternal Flower... found in the game but never relesed (poor thing we will have to wait 3000 years to get it XD)
Last... Hoopa Unbound found in the ORAS demo

All this to say that that when a demo/full game is released they can get all the dex/moves/3D sprites before the official reveal/artworks

I'd like to believe that this pokemon is just a marketing tool. Kinda like how zygarde cells and cores were teased at the end of a movie last time.
Zygarde cells can't be used in battle in the main series. Somehow, I wish this Pokemon will be the same.

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There's little information for now... it can be a placeholder (I doubt it since this thing is well made: shadows, animations, shiny form) or the official Pokémon that can close Gen 7 or be the first one of Gen 8... or simply a marketing move to create some new interest for GO... we have to wait something official...